Baryon Acoustic Oscillations: A standard ruler method for determining the expansion rate of the Universe. Martin White UC Berkeley/LBNL Outline • Dark energy and standard rulers. • Cosmic sound: baryon acoustic oscillations. • Current state-of-the-art • Future experiments. • More on theoretical issues. • More on modeling issues. • Prospects and conclusions. Eisenstein, New Astronomy Reviews, 49, 360, 2005 http://cmb.as.arizona.edu/~eisenste/acousticpeak/ http://cdm.berkeley.edu/doku.php?id=baopages Beyond Einstein? Our theories of the Universe are based upon General Relativity which, like Newton’s theory, predicts that gravity is an attractive force which would act to slow any existing expansion. 1919 1998 The discovery that the expansion of the Universe is currently accelerating was heralded as the “Breakthrough of the year” by Science in 1998. Dark energy • There are now several independent ways to show that the expansion of the Universe is accelerating. • This indicates that: a) Our theory of gravity (General Relativity) is wrong. b) The universe is dominated by a material which violates the strong energy condition: ρ+3p>0. • If (b) then it cannot be any fluid we are familiar with, but some weird “stuff” which dominates the energy density of the Universe (today). We refer to it as “dark energy”. • The most prosaic explanation is Einstein’s cosmological constant, which can be interpreted as the energy of empty space. Dark energy equation of state • The amount of dark energy is actually quite well constrained by present data: -29 3 ρDE = (1.43±0.09)x10 g/cm • What distinguishes models is the time-evolution of ρDE • This is usually described by the equation of state: w=p/ρ. – A cosmological constant, vacuum energy, has w=-1. – Many (most) dark energy models have w>-1, and time evolving. • So the “holy grail” of DE research is to demonstrate that w ≠-1 at any epoch. Probing DE via cosmology • We “see” dark energy through its effects on the expansion of the universe: • Three (3) main approaches – Standard candles -1 • measure dL (integral of H ) – Standard rulers -1 • measure dA (integral of H ) and H(z) – Growth of fluctuations. • Crucial for testing extra ρ components vs modified gravity. Standard rulers • Suppose we had an object whose length (in meters) we knew as a function of cosmic epoch. • By measuring the angle (Δθ) subtended by this ruler (Δχ) as a function of redshift we map out the angular diameter distance dA • By measuring the redshift interval (Δz) associated with this distance we map out the Hubble parameter H(z) Ideal properties of the ruler? To get competitive constraints on dark energy we need to be able to see changes in H(z) at the 1% level -- this would give us “statistical” errors in DE equation of state (w=p/ρ) of ~10%. • We need to be able to calibrate the ruler accurately over most of the age of the universe. • We need to be able to measure the ruler over much of the volume of the universe. • We need to be able to make ultra-precise measurements of the ruler. Where do we find such a ruler? • Cosmological objects can probably never be uniform enough. • We believe that the laws of physics haven’t changed over the relevant time scales. – Use features arising from physical processes in the early Universe. • Use statistics of the large-scale distribution of matter and radiation. – If we work on large scales or early times perturbative treatment is valid and calculations under control. Sunyaev & Zel’dovich (1970); Peebles & Yu (1970); Doroshkevitch, Sunyaev & Zel’dovich (1978); …; Hu & White (1996); Cooray, Hu, Huterer & Joffre (2001); Eisenstein (2003); Seo & Eisenstein (2003); Blake & Glazebrook (2003); Hu & Haiman (2003); … Back to the beginning … The CMB power spectrum Angular scale The current CMB data are in excellent agreement with the theoretical predictions of a Anisotropy power ΛCDM model. Hinshaw et al. (2008) The cartoon • At early times the universe was hot, dense and ionized. Photons and matter were tightly coupled by Thomson scattering. – Short m.f.p. allows fluid approximation. • Initial fluctuations in density and gravitational potential drive acoustic waves in the fluid: compressions and rarefactions with δγ∝δb. • Consider a (standing) plane wave perturbation of comoving wavenumber k. • If we expand the Euler equation to first order in the Compton mean free path over the wavelength we obtain The cartoon • These perturbations show up as temperature fluctuations in the CMB. • Since ρ~T4 for a relativistic fluid the temperature perturbations look like: [harmonic wave] • … plus a component due to the velocity of the fluid (the Doppler effect). The cartoon • A sudden “recombination” decouples the radiation and matter, giving us a snapshot of the fluid at “last scattering”. • These fluctuations are then projected on the sky with λ~rlsθ or l~k rls Acoustic oscillations seen! First “compression”, at kcstls=π. Density maxm, velocity null. Velocity maximum First “rarefaction” peak at kcstls=2π Acoustic scale is set by the sound horizon at last scattering: s = cstls CMB calibration • Not coincidentally the sound horizon is extremely well determined by the structure of the acoustic peaks in the CMB. WMAP 5th yr data Dominated by uncertainty in ρm from poor constraints near 3rd peak in CMB spectrum. (Planck will nail this!) Baryon oscillations in P(k) • Since the baryons contribute ~15% of the total matter density, the total gravitational potential is affected by the acoustic oscillations with scale set by s. • This leads to small oscillations in the matter power spectrum P(k). – No longer order unity, like in the CMB, now suppressed by Ωb/Ωm ~ 0.1 • Note: all of the matter sees the acoustic oscillations, not just the baryons. Baryon (acoustic) oscillations fluctuation RMS Wavenumber Divide out the gross trend … A damped, almost harmonic sequence of “wiggles” in the power spectrum of the mass perturbations of amplitude O(10%). In configuration space • The configuration space picture offers some important insights,and will be useful when we consider non-linearities and bias. • In configuration space we measure not power spectra but correlation 2 functions: ξ(r )=∫Δ (k)j0(kr) dlnk. • A harmonic sequence would be a δ-function in r, the shift in frequency and diffusion damping broaden the feature. Acoustic feature at ~100 Mpc/h with width ~10Mpc/h (Silk scale) Configuration space In configuration space on uses a Green’s function method to solve the equations, rather than expanding k-mode by k- mode. (Bashinsky & Bertschinger 2000) To linear order Einstein’s equations look similar to Poisson’s equation relating φ and δ, but upon closer inspection one finds that the equations are hyperbolic: they describe traveling waves. [effects of local stress-energy conservation, causality, …] In general the solutions are unenlightening, but in some very simple cases you can see the main physical processes by eye, e.g. a pure radiation dominated Universe: The acoustic wave Start with a single perturbation. The plasma is totally uniform except for an excess of matter at the origin. High pressure drives the gas+photon fluid outward at speeds approaching the speed of light. Baryons Photons Mass profile Eisenstein, Seo & White (2006) The acoustic wave Initially both the photons and the baryons move outward together, the radius of the shell moving at over half the speed of light. Baryons Photons The acoustic wave This expansion continues for 105 years The acoustic wave After 105 years the universe has cooled enough the protons capture the electrons to form neutral Hydrogen. This decouples the photons from the baryons. The former quickly stream away, leaving the baryon peak stalled. Baryons Photons The acoustic wave The photons continue to stream away while the baryons, having lost their motive pressure, remain in place. The acoustic wave The acoustic wave The photons have become almost completely uniform, but the baryons remain overdense in a shell 100Mpc in radius. In addition, the large gravitational potential well which we started with starts to draw material back into it. The acoustic wave As the perturbation grows by ~103 the baryons and DM reach equilibrium densities in the ratio Ωb/Ωm. The final configuration is our original peak at the center (which we put in by hand) and an “echo” in a shell roughly 100Mpc in radius. Further (non-linear) processing of the density field acts to broaden and very slightly shift the peak -- but galaxy formation is a local phenomenon with a length scale ~10Mpc, so the action at r=0 and r~100Mpc are essentially decoupled. We will return to this … Features of baryon oscillations • Firm prediction of models with Ωb>0 • Positions well predicted once (physical) matter and baryon density known - calibrated by the CMB. • Oscillations are “sharp”, unlike other features of the power spectrum. • Internal cross-check: -1 – dA should be the integral of H (z). • Since have d(z) for several z’s can check spatial flatness: “d(z1+z2) = d(z1)+d(z2)+O(ΩK)” • Ties low-z distance measures (e.g. SNe) to absolute scale defined by the CMB (in Mpc, not h-1Mpc). – Allows ~1% measurement of h using trigonometry! Aside:broad-band shape of P(k) • This picture also allows us a new way of seeing why the DM power spectrum has a “peak” at the scale of M-R equality. • Initially our DM distribution is a δ-function. • As the baryon-photon shell moves outwards during radiation domination, its gravity “drags” the DM, causing it to spread. • The spreading stops once the energy in the photon- baryon shell no longer dominates: after M-R equality. • The spreading of the δ-function ρ(r) is a smoothing, or suppression of high-k power.